1. Field of the Invention
The present invention relates to bulk goods transport apparatus, and particularly describes a conveyor belt monitor for detecting loosened belt splices, tears, and other irregularities in conveyor belts before the belt breaks and shuts down the conveyor.
2. Description of Related Art
Conveyor machinery for transporting bulk goods, packaged goods, and other items are well known and appear in a variety of forms, depending on their application. The present invention relates to belt conveyors which may be used, for example, in the coal mining industry to transport bulk coal up an incline from a coal seam in an underground mine to a point where it may be discharged either onto another conveyor belt or into a rail car. Such a conveyor belt may span a length of 1,500 feet to 5,000 feet.
A typical belt conveyor consists of a supporting structure or frame; a continuous or endless belt of fabric made from leather, rubber, or leather and cotton duck impregnated with rubber and built up in plies with a rubberized covering; a plurality of cylindrical pulleys or drums to support and tension the belt, including a head drum, a tail drum and one or more idler drums; and a drive system, often a motor with gearing or a chain and sprocket assembly attached to a drive drum, which may also be the head drum.
Various problems arise with maintenance of the fabric belt occurring from wear and tear and the rough usage conditions associated with operation under difficult environmental conditions. Fabric belts may develop tears, which are repaired by joining a seam running across the belt, either by means of a plurality of metal splices having teeth penetrating the fabric and catching on opposing ends of the seam, often secured by rivets, or by a vulcanizing splice using an adhesive secured by heating the joint to secure the opposing ends of the seam. Over time the metal splices may become loosened, projecting out of the belt and weakening the belt, or the vulcanized splice may have a ply of the belt curl back and tear from repeated friction. If either of these conditions are not discovered by ordinary maintenance, they will eventually cause the belt to break, dropping the load of bulk goods and shutting down the conveyor until repairs can be made and the spillage cleaned up. When the breakage occurs in the seam of an underground mine with a low ceiling, repair and clean up operations can cause extended interruption of normal mining operations. Clearly early warning of an impending break in the belt is desirable.
Other problems that can occur include twisting of the belt, misalignment of the belt, and slippage. Twisting of the belt can occur from foreign objects, such as wood and metal scraps, becoming lodged temporarily at the tailpiece or head, causing one side of the belt to move faster than the other, and eventually the belt twists so the one side partially overlaps the other. Misalignment can occur from the same cause, with one side of the belt shifting off the drums, shifting increasing due to the weight of the cargo load, causing spillage of bulk materials onto the return loop, with eventual clogging of the drums and twisting or longitudinal tearing of the belt. Slippage occurs due to lengthening of the belt due to constant tensile and shear forces on the flexible belt.
A variety of apparatus and devices have been proposed to remedy these problems. U.S. Pat. No. 2,345,415, issued Mar. 28, 1944 to Andrew Nagy, Jr., describes a limit switch activated by a pivoting lever having a support arm attached to a plate extending across the width of the belt, positioned vertically above the belt and with very narrow clearance between the belt and the plate, so that a clamp or splice projecting from the top surface of the belt will rotate the lever and actuate the switch, shutting down the conveyor before the belt breaks.
U.S. Pat. No. 4,462,523, issued Jul. 31, 1984 to J. F. Kerr, describes a belt monitor having a cable extending below the belt from one side to the other, the cable being anchored on one side of the belt and attached to a ball and socket mechanism on the other side of the belt. When a damaged portion of a torn belt is encountered, the cable pulls the ball from the socket, causing a spring biased cam to move towards the socket, thereby tripping a pair of microswitches. One monitor may be placed below the belt on the upper run and a second monitor may be placed below the belt on the return run. At each location, the monitor must be used in pairs, one with the cable anchored at one edge of the belt, the other with the cable anchored at the other side of the belt, since a single cable only detects tears between the detector and the midline of the belt. The monitor must be rest manually by reinserting the ball in the socket after repair of the cable.
German Patent No. 4,011,257, published Jan. 31, 1991, shows a device for detecting longitudinal tears in the belt, including a pressure drum with rollers mounted at either end of the drum, the drum tapering narrowly towards the center of its axis and movable perpendicular to the direction of belt movement, tears being detected by a difference in the pressure exerted by the rollers which shuts down the machine. Soviet Invention Certificate No. 1,542,873, published Feb. 15, 1990, teaches another device for controlling longitudinal tears, having a pivotally mounted crank with two arms having rollers at the ends of the arms which are in contact with the return loop of the belt, longitudinal tears being detected as a difference in the pressure of the rollers and sounding an alarm signal.
Japanese Patent No. 6-24538, published Feb. 1, 1994, discloses a device for detecting a twisted belt having two pairs of parallel rollers mounted at a drum, the twist being detected as a difference in the radial height of the rollers. Russian Invention Certificate No. 1,033,401, published Aug. 7, 1983, describes a conveyor belt having a flexible element extending above and parallel to the belt attached to a pair of eccentrically mounted trapping drums which have projections on their surface which are positioned in close approximation to the top of the belt surface. An irregularity in the belt causes the tail roller to slide down, actuating the flexible element, causing the projections of the trapping drum to catch the belt against a plate and stop the conveyor until a stop switch can be manually operated to disconnect the drive drum.
Other conveyor safety devices and monitors are described in U.S. Pat. No. 4,564,099, issued Jan. 14, 1986 to Akira Uozumi (safety device for an escalator or other passenger conveyor warning of a difference between handrail speed and tread speed); U.S. Pat. No. 5,647,640, issued Jul. 15, 1997 to Heintzmann, et al. (device to correct sag in a chain conveyor by adjusting drive speed); and German Patent No. 2,017,949, published Dec. 28, 1971 (device to maintain measured amount of slack in chain conveyor by adjusting drive speed).
Despite the advances described in the above patents, there remain problems to be resolved. The Nagy device, for example, is mounted directly above a pulley, probably because that is the only flat portion of the upper belt path, and will therefore activate the limit switch and stop the conveyor whenever there is a load on the belt, and with bulk material, a bad splice would be covered by the bulk material and not detected. Further, the Nagy device only checks for irregularities on the outer surface of the belt; many tears begin on the inner surface of the belt. The Nagy device must be manually reset because of the construction of the lever and support arm; it would be desirable to have a device which automatically resets so the operator may restart the conveyor to move the irregularity to a convenient repair position, which may be the other end of the conveyor span. Belt twists often occur as the belt goes around the pulley; it is desirable to position the device on the return loop after the belt has gone around the pulley.
The device described by the Kerr patent must be also be manually reset, by reinserting the ball in the socket. The Kerr device also lacks sensitivity, since the cable must be set sufficiently low to account for sag in the belt caused by the load, and since the top and the bottom of the belt are not monitored simultaneously. Hence a tear in the belt may ride over the tear until the belt is at the point of rupture.
The German '257 patent and the Soviet '873 patents depend on rollers which press against the belt. Belt conveyors operate at speeds between 550 and 900 feet per minute. Such rollers are inclined to wear out quickly and form sharp edges, which might tear the belt. The rollers would also require high quality roller bearings which would need frequent greasing.
Consequently, an apparatus which overcomes the limitations of conventional devices is needed. None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.